An electrostatographic image is produced by generating an electrostatic latent image on a primary image forming member. A visible image is then produced by bringing the electrostatic latent image into close proximity to an appropriate developer. The image is then transferred to a receiver and permanently fixed to that receiver by a suitable process such as fusing. If the electrostatographic process is electrophotographic, the primary image forming member comprises a photoconductive member. The photoconductive member is initially uniformly charged. The electrostatic latent image is produced by image-wise exposing the charged photoconductive member to an exposure source such as an optical exposure means, LED array, laser-scanner, or other electro-optical exposure device. The latent image is then developed by bringing the latent-image bearing photoconductive member into close proximity to an appropriate developer comprising electrically charged marking or toner particles. The image is then transferred from the photoconductor to an appropriate receiver such as paper or transparency stock. Although transfer can be effected using a variety of means, it is generally accomplished by applying an electrostatic potential to urge the toner particles from the photoconductive member to the receiver. Alternatively, the image can be transferred first to an intermediate member and subsequently to the receiver. The image is then permanently fixed to the receiver using suitable means such as applying heat and pressure to melt the toner in a process known as fusing. The photoconducting member is then cleaned and made ready to produce subsequent images.
It is well known that the adhesive and cohesive properties of toner particles affect transfer. The term, "adhesive", refers to attractive forces between particles and a receiver surface. The term, "cohesive", refers to attractive forces between similar particles. Specifically, as the toner diameter decreases, the forces holding the toner particles to surfaces such as the primary imaging member start to dominate over the electrostatically applied transfer force. For all practical purposes, this occurs for toner particles without particulate addenda when the toner diameter is less than approximately 12 .mu.m (micrometers).
There have been numerous methods employed to facilitate toner transfer for toner particles having diameters less than 12 .mu.m. For example, toned images have been transferred thermally. However, this often requires specific receivers and can be harsh on the primary imaging members, especially photoconductors. Release agents such as zinc stearate have been applied to primary imaging members. However, these often interact with the charging properties of the toner particles in undesirable fashions. Moreover, they do not last on the primary imaging member and need to be replenished. This often requires complex subsystems and process control. In another method of reducing toner adhesion to the primary imaging member, the surface of the toner is coated with sub-micrometer particulate addenda such as silica particles. These addenda often do not form a uniform coating on the toner particles, but, rather, agglomerate into clusters having cluster diameters in the range of tens of nanometers, as determined using scanning electron microscopy (SEM). Using this technology, it has been possible to reduce the volume weighted toner diameter, wherein the adhesion forces holding the toner to the primary imaging member dominate over the applied electrostatic transfer force, from approximately 12 .mu.m to approximately 8.5 .mu.m. However, it is unlikely that a further decrease in toner size using this technology alone would be feasible.
In another method of electrostatically transferring toner particles, Rimai and Chowdry in U. S. Pat. No. 4,737,433 have shown that, by using monodisperse, spherical toner particles and smooth receivers, it is possible to balance the surface forces, thereby permitting electrostatic transfer of toner particles having diameters as little as 2 .mu.m. However, particulate contaminants such as dust, carrier particles, etc. separate the receiver from the primary image forming member, thereby creating artifacts in the image. Moreover, the requirement that one use very smooth receivers limits the utility of this technique.
Another method of transfer employs the use of a compliant intermediate transfer member. In this method of transfer, the toned image is first transferred from the primary image forming member to the compliant intermediate. The image is subsequently transferred from the intermediate to the receiver. In a preferred mode of operation and with reference to International Published Application WO 98/04961, color images are produced by transferring the toned color separation images from the primary image forming member to the compliant intermediate in register and then transferring the entire image to the receiver. In another preferred embodiment, the color separation images can be produced in separate respective color modules wherein each color separation image is transferred to a separate respective compliant intermediate. The images are then transferred sequentially from the respective intermediates, in register, to the receiver. In a less preferred embodiment, the various color separation images could be transferred sequentially to a single compliant intermediate member and alternately transferred in register to the final receiver surface.
The use of a compliant intermediate member may permit balancing of surface forces. Indeed, Zaretsky and Gomes (U.S. Pat. No. 5,370,961) have shown that it is possible to transfer images made with silica-coated toner particles having diameters of 3.5 .mu.m using compliant intermediates.
It is often not desirable to use toner particles as small as those used by Zaretsky and Gomes because development rates decrease with decreasing toner size. Moreover, for many applications, such as in binary imaging, wherein the image consists of halftone dots, multibit level dots, alpha-numerics, lines and text, etc., very small particles (i.e. those having diameters less than 5 .mu.m) may not give substantial improvements in image quality. Nonetheless, it is often desirable to use toner particles having diameters less than 10 .mu.m and even more desirable to use toner particles having diameters between 5 .mu.m and 9 .mu.m. To do so it is necessary to transfer such images with high efficiency but without significant degradation of the toned image.
Degradation in transfer often occurs because the electrostatically charged toner particles tend to repel each other. However, cohesive forces between the particles tend to stabilize the toned image structure. However, as adhesion is decreased by the addition of the particulate addenda, cohesion is also reduced, thereby aggravating image disruption and resulting in toner particles forming satellites around the image. This causes objectionable background and results in other artifacts such as a loss of resolution and sharpness.
The reduction of cohesion between toner particles themselves can introduce new problems during transfer. As the images, comprised of collections of charged toner particles, are transferred to the receiver, the repulsive electrostatic forces between toner particles can cause the images to fly apart. This effect is most apparent in halftone dot images where the halftone dots literally can explode. While dot explosions can occur in non-treated toner systems, it has been observed that the use of submicrometer particulate addenda can aggravate the dot explosion problem, presumably by reducing the cohesion between toner particles and thereby accentuating the electrostatic repulsion between those particles. Alternatively, it is possible that when transfer is accomplished using an electrically biased transfer nip, dot explosion may be caused by transfer of some of the surface-treated toner particles and halftone dots across the air gap in the pre-nip region due to high electrostatic fields. Sufficiently large electrostatic fields produced in this pre-nip region, can destabilize the fragile dots that are held together by surface forces. The cohesive forces must overwhelm the electrostatic repulsion between the like sign charged toner particles in order to keep the dots from exploding. If transfer occurs only after the photoconductor is in physical contact with the receiver, the effects of dot explosion can be reduced since the toner particles, including those which might otherwise become satellites, will not be able to move very far from their intended location.
Improvement in transfer efficiency with minimal image disruption represents an important problem in the field of electrostatography.